The development of the ocular lens involves controlled proliferation and progressive differentiation of equatorial epithelial cells into terminally differentiated lens fibers. Intracellular signaling mechanisms that regulate these cellular processes and their possible role in cataractoqenesis have not been characterized. We hypothesize that Rho GTPases (Rho and Rac) play a critical role in lens growth and development and in maintenance of lens structural integrity by regulating cytoskeletal organization and growth factor stimulated stress-response signaling pathways. Strong evidence for this hypothesis derives from our recent studies based on pharmacological and genetic modulation of Rho GTPase function in different model systems including lens organ and epithelial cell culture, and a C3-exoenzyme transgenic mouse model. Inactivation of Rho GTPase in cell culture and in vivo resulted in alterations of lens epithelial and fiber cell morphology, actin cytoskeletal orqanization and cell adhesive characteristics. Additionally, the Rho GTPase functional knock-out mouse developed cataract and exhibited differential expression of genes encodinq proteins of the extracellular matrix and basement membrane, as well as proteins of stress signaling and apoptotic pathways. Further, several growth factors activated Rho GTPases (both Rho and Rac) in lens epithelial cells. These observations support a critical role for Rho GTPase (Rho and Rac)-mediated signaling mechanisms in lens development, growth and integrity. To elucidate the mechanistic basis (es) by which Rho GTPases regulate lens growth and development, we propose to investigate 1. The roles of RhoA and RhoB GTPases and their down stream effector-Rho kinase, in lens epithelial cell proliferation and cell fate using a RhoB knockout mouse, and by overexpressing dominant neqative mutants of RhoA or Rho kinase in a human lens epithelial cell line using adenoviral vectors. 2. The role of Rac GTPase and reactive oxygen species (ROS)-activated stress signaling pathways (C-jun-N-terminal kinase-JNK1 and p38 MAPK) in growth factor-induced proliferation and cell survival (Akt and NFkappaB) of human lens epithelial cells using an NADPH oxidase inhibitor and by overexpression of dominant negative and constitutively active mutants of Rac GTPase. 3. Regulation of lens protein tyrosine phosphatase activity by Rac/NADPH oxidase in organ-cultured lenses and in lens epithelial cells, in the context of lens growth and epithelial cell proliferation. The completion of these studies should unravel the significance of Rho/Rho kinase and Rac GTPase-mediated signaling and the role of stress -activated signaling pathways in proliferation, survival and apoptosis of lens epithelial cells. Better understanding of signal transduction pathways regulating lens growth and maintenance of lens transparency could provide insights into the etiology of cataract formation and into novel approaches towards developing medical treatments for certain types of cataract.